Line head and image forming apparatus incorporating the same

ABSTRACT

In a line head, a plurality of element arrays arranged in a first direction. Each array includes a plurality of light emission elements arrayed in a second direction which is perpendicularly to the first direction. The light emission elements emit light for forming an electrostatic latent image on a photosensitive surface of an image carrier. A switcher activates the light emission elements in at least one of the element arrays while deactivating the others. A developer develops the latent image as a visible image with toner.

BACKGROUND OF THE INVENTION

The present invention relates to a line head in which a plurality oflight emission elements are arrayed is used as an exposer for forming alatent image to be developed, which is incorporated in an image formingapparatus using an electrophotographic process.

Japanese Patent Publication No. 4-363264A discloses a line head in whicha plurality of electroluminescence (EL) elements are arrayed to form asingle line, and teaches that an interval between pulses for driving theelements to emit light is controlled so as to elongate the life of theelements. Japanese Patent Publication No. 3-101366A discloses a linehead in which a number of EL elements are arrayed to form a single line,one auxiliary pulse for driving all the elements to emit light isnecessarily incorporated irrespective of main driving pulses in eachprimary scanning operation, so that predetermined light intensity can beobtained in a short time even when the devices have not emitted lightfor a long time.

FIG. 20 shows one example of a time-to-luminance characteristic of an ELelement. The abscissa is used for a time (minutes) and the ordinate isused for luminance. As is shown, luminance decreases with an elapse oftime since the EL element has activated (by about 20% in five minutes).The luminance is restored if the EL element is deactivated for apredetermined time period to decrease the element temperature, and isactivated again. To simplify the explanation, the deactivated period isomitted from FIG. 20.

Each of the line heads disclosed in the above publications is providedwith a single array of the EL elements. Hence, if such a line head issubjected to continuous activation for a long time period, the luminancedecreases as shown in FIG. 20, thereby deteriorating the quality of animage to be obtained. In addition, since the degree of the luminancedecrease in one element is actually different from another element, thedeterioration of the obtained image quality becomes remarkable after thelong-term activation. Further, since the life of the EL element isshortened by such a continuous activation, the replacement of the linehead is hastened.

Moreover, at the occurrence of an unexpected event due to a failure inany of the light emission elements or the like, the line head has to bereplaced, which poses a problem that the replacement is tedious and thecost is thereby increased.

SUMMARY OF THE INVENTION

It is therefore an object or the invention to provide a line headcapable of addressing such an unexpected event, avoiding such imagedeterioration due to the long-term activation, and prolonging thelifetime thereof.

It is also an object of the invention to provide an image formingapparatus incorporating such a line head.

In order to achieve the above objects, according to the invention, thereis provided a line head, comprising:

a plurality of element arrays arranged in a first direction, each arrayincluding a plurality of light emission elements arrayed in a seconddirection which is perpendicularly to the first direction; and

a first switcher, which activates the light emission elements in atleast one of the element arrays while deactivating the others.

Preferably, at least one of the element arrays is used for a backuppurpose. In this case, the image forming operation can be continuedwithout replacing the line head, even if a failure is occurred on one ofthe element arrays.

Preferably, each of the element arrays is divided into a plurality ofblocks; and the line head further comprises a second switcher whichactivates the light emission elements in at least one of the blocks inthe at least one element array activated by the first switcher.

In this case, since only the block in the element array can be activatedin accordance with the required area of the image formation, economiclight emission can be realized.

Here, it is preferable that a third switcher activates at least one ofthe light emission elements in the at least one block activated by thesecond switcher.

In this case, the activation/deactivation and the activated time periodof the respective light emission elements can be independentlycontrolled.

Alternatively, the second switcher may activate at least one of thelight emission elements in the at least one element array activated bythe first switcher.

Preferably, each of the light emission elements is provided as either anorganic electroluminescence element or a light emitting diode.

In the former case, since the organic electroluminescence element can becontrolled statically, the control system can be simplified. In thelatter case, the manufacturing process for the light emission elementcan be simplified.

Here, it is preferable that the first switcher controls a potential atone of a cathode or an anode of each of the light emission elements.Here, a potential at the other one of the cathode or the anode is fixed.In this case, the switching operation can be simplified.

Preferably, the first switcher comprises switching transistors. In thiscase, the switching between the element arrays can be executed rapidlyand accurately.

Here, it is preferable that: each of the element arrays is associatedwith one of the switching transistors; and the first switchersimultaneously generates a first signal for activating the switchingtransistor and a second signal for deactivating the switchingtransistor; so that the first signal is inputted to the switchingtransistor for the at least one element arrays to be deactivated, whilethe second signal is inputted to the switching transistor for the othersto be deactivated.

In this case, the signal generation for the switching between theelement arrays can be simplified.

Alternatively, it is preferable that the element arrays includes atleast one first element array and at least one second element array; theswitching transistors includes a first transistor associated with thefirst element array and a second transistor associated with the secondelement array and having a conductive type which is opposite to thefirst transistor; and the first switcher inputs a common signal to thefirst transistor and the second transistor, so that one of the firsttransistor and the second transistor is activated while the other one isdeactivated.

In this case, the wiring for providing the signals for switching betweenthe element arrays can be simplified.

Preferably, the first switcher sequentially activate at least one of theelement arrays when a predetermined requirement is satisfied.

In this case, the image forming operation can be continued withoutreplacing the line head, even if the luminance of the light emissionelement in the activated element array decreases due to the long-termcontinuous activation.

Preferably, the first switcher sequentially activate at least one of theelement arrays with a predetermined order.

In this case, since each of the element arrays intermittently activated,not only the luminance decrease of the light emission element due to thelong-term continuous activation can be avoided, but also the lifetime ofthe line head can be prolonged.

Preferably, the first switcher sequentially activate at least one of theelement arrays in accordance with an image to be formed.

In this case, for example, the element array in which the luminance ofthe light emission elements decreases can be used for an image with lessdata such as a binary date image. Accordingly, not only thedeterioration of the obtained image quality can be avoided, but also thelifetime of the line head can be prolonged.

Here, it is preferable that the element arrays includes at least onefirst element array for forming a binary data image, and at least on asecond element array for forming a gradation data image.

In this case, the element array in which the luminance of the lightemission elements decreases can be used to form the binary data image tomaintain the obtained image quality.

Alternatively, it is preferable that the element arrays includes atleast one first element array capable of forming both of a binary dataimage and a gradation image, and at least one second element array forforming only the gradation data image.

In this case, the restoration of the decreased luminance of the lightemission elements in the first element array can be secured withoutstopping the image forming operation because the first array may be usedto form the binary data image until the luminance is restored to a levelenough to form the gradation data image.

According to the invention, there is also provided an image formingapparatus, comprising:

an image carrier, having a photosensitive surface;

a line head, comprising:

-   -   a plurality of element arrays arranged in a first direction,        each array including a plurality of light emission elements        arrayed in a second direction which is perpendicularly to the        first direction, to emit light for forming an electrostatic        latent image on the photosensitive surface; and    -   a first switcher, which activates the light emission elements in        at least one of the element arrays while deactivating the        others; and

a developer, which develops the latent image as a visible image withtoner.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and advantages of the present invention will becomemore apparent by describing in detail preferred exemplary embodimentsthereof with reference to the accompanying drawings, wherein:

FIG. 1 is a diagram for explaining an operation of a line head accordingto a first embodiment of the invention;

FIG. 2 is a block diagram of a peripheral configuration of the line headof FIG. 1;

FIG. 3 is a circuit diagram of the line head of FIG. 1;

FIG. 4 is a circuit diagram of a line head according to a secondembodiment of the invention;

FIG. 5 is a circuit diagram of a line head according to a thirdembodiment of the invention;

FIG. 6 is a circuit diagram of a line head according to a fourthembodiment of the invention;

FIG. 7 is a circuit diagram of a line head according to a fifthembodiment of the invention;

FIG. 8 is a circuit diagram of a line head according to a sixthembodiment of the invention;

FIG. 9 is a circuit diagram of a line head according to a seventhembodiment of the invention;

FIG. 10 is a diagram of a line head according to an eighth embodiment ofthe invention;

FIG. 11 is a diagram for explaining an operation of the line head ofFIG. 10;

FIG. 12 is a circuit diagram of a line head according to a ninthembodiment at the invention;

FIG. 13 is a time chart for explaining an operation of the line head ofFIG. 12;

FIG. 14 is a table for explaining an operation of the line head of FIG.12;

FIG. 15 is a diagram for explaining an operation of a line headaccording to a tenth embodiment of the invention;

FIG. 16 is a diagram for explaining an operation of a line headaccording to an eleventh embodiment of the invention;

FIG. 17 is a block diagram of a peripheral configuration of the linehead of FIG. 15 or 16;

FIGS. 18 and 19 are schematic views showing examples of an image formingapparatus incorporating the line head according to any one of the aboveembodiments; and

FIG. 20 is a graph showing a time-luminance characteristic of anelectroluminescence element.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will be described below in detail withreference to the accompanying drawings.

As shown in FIG. 10, in a line head 10 according to a first embodimentof the invention, a number of light emission elements La are arrayed ina primary scanning direction (X direction) to form an element array, anda plurality of elements arrays 1 a through 1 d are arranged in asecondary scanning direction (Y-direction). Beside the EL element, alight emitting diode (LED) can be adopted as the light emission elementLa.

In this embodiment, the element arrays 1 b through 1 d are provided asthe element arrays for a backup operation. When the light emissionelements La in the element array 1 a for a normal operation have beenactivated for a predetermined time, the element array 1 b for a backupoperation is caused to emit light.

As described the above, the element array provided for the backupoperation is not limited to one array. The element array 1 c can be usedas the backup array for the element array 1 b. Similarly, the elementarray 1 c can be used as the backup array for the element array 1 c. Thearrays 1 c and 1 d may be used for arrays for performing multipleexposure.

The line head of the invention is configured such that at least onearray among a plurality of arrays arranged in the secondary scanningdirection is used for the backup operation. In a case where three arraysof EL elements are provided, at least one array is used for the backupoperation. That is, one array may be used as a normal operation whiletwo arrays may be used as the backup operation. Alternatively, twoarrays may be used as the normal operation while one array may be usedas the backup operation.

FIG. 1 shows how to operate the line head 10 in this embodiment. Inorder to simplify the explanation, there will be described a case whereonly the two arrays 1 a and 1 b are operated. In this case, all thelight emission elements in the element array 1 a are activated from thetime points 0 to ta. All the light emission elements in the elementarray 1 b are deactivated. At the time point ta, the element array 1 ais deactivated, and switching is made for the element array 1 b to beactivated. Thereafter, the element array 1 a and the element array 1 bare activated alternately at the time points tb, tc, td, te, and tf.

Since the two element arrays are alternately activated at apredetermined cycle so that each element array is intermittentlyactivated, the luminance decrease due to the long term continuousactivation can be avoided. Hence, not only can deterioration of theobtained image quality be prevented, but also the life of the line headcan be prolonged.

The switching timings for the respective element arrays, that is, theactivation time period and the deactivation time period, are set bytaking various factors into account. In other words, optimal timings areset on the basis of a temperature characteristic of light emissionelement materials, a heat-releasing characteristic of a panel of theline head to which the light emission elements are attached, etc.

As shown in FIG. 2, the line head 10 is incorporated in a controlsection 20 of an image forming apparatus. A host controller 23 forgenerating image data, which is constituted by a computer, for example,is connected to the image forming apparatus. The control section 20comprises a switching circuit 21, a counter 24, and a memory 25 inaddition to the line head 10. The counter 24 counts the activated timeperiod of each element array and stores the counted value into thememory 25. The memory 25 stores the prescribed switching timing inadvance.

The host controller 23 judges whether the activated time period of oneelement array reaches the switching timing stored in the memory 25. Ifit reaches, a switching signal is transmitted to the switching circuit21 to deactivate the one element array while activating another elementarray in the line head 10.

In this embodiment, the host controller 23 controls the counter 24 andthe memory 25. However, a controller such as a CPU for controlling thecounter 24 and the memory 25 may be provided in the control section 20,such that the controller is formed on a substrate on which the switchingcircuit 21 is provided. In this case, the configuration of the controlsystem can be compact and the host controller 23 is not involved. Hence,the wiring can be shorter, which can in turn increase the processingspeed.

FIG. 3 show a circuit for switching the element array to be activated.The line head 10 is provided with element arrays 1 a and 1 b. In theelement array 1 a are arrayed light emission elements D00 through D23each comprising, for example, an EL element. Also, in the element array1 b are arrayed light emission elements D50 through D73 each comprisingan EL element.

A positive power supply line 4 is commonly constructed to anodes of therespective light emission elements in the element arrays 1 a and 1 b. Anegative power supply line 5 is connected to cathodes of the respectivelight emission elements in the element array 1 a, and a negative powersupply line 6 is connected to cathodes of the respective light emissionelements in the element array 1 b. In other words, the element array 1 ais connected between the power supply lines 4 and 5, so that a DCvoltage is applied. Also, it is configured in such a manner that theelement array 1 b is connected between the power supply lines 4 and 6,so that a DC voltage is applied. It should be noted that in the actualline head 10, control circuits to activate individual light emissionelements may be provided between the anodes of the respective lightemission elements and the positive power supply line 4.

When a contact 3 c of a switch 3 is placed on a contact point 3 a, a DCvoltage is applied between the power supply lines 4 and 5, which causesthe respective light emission elements D00 through D23 in the elementarray 1 a to activate. When the contact 3 c of the switch 3 is placed ona contact point 3 b, a DC voltage is applied between the power supplylines 4 and 6, which causes the respective light emission elements D50through D73 in the element array 1 b to activated.

The element array 1 a is provided for a normal operation, and theelement array 1 b is provided for a backup operation. In the event thatluminance of the element array 1 a decreases, a voltage is applied tothe respective light emission elements D50 through D73 in the elementarray 1 b by the switch 3 so that they are activated. That is, in thisembodiment, the element arrays are switched by the switch 3, byswitching the power supply lines 5 and 6, to which are commonlyconnected the cathodes of the light emission elements in the respectiveelement arrays. In this instance, the positive power supply line 4 iscommonly connected to the anodes of the light emission elements in therespective element arrays. In other words, because only the power supplylines connected to one polarity of the light emission elements areswitched, the configuration of the switching circuit can be simpler thanin a case where the power supply lines of the both polarities areswitched.

The switch 3 is embodied by a mechanical switch as described the above,however, it may be embodied by an electronic switch such as atransistor. Either one of the element array 1 a and the element array 1b is used for the normal operation and the other for the backupoperation, and it is therefore possible to use the element array 1 b forthe normal operation and the element array 1 a for the backup operation.

FIG. 4 shows a second embodiment of the invention. The same componentsare designated by the same reference numerals and repetitiveexplanations will be omitted.

In a line head 10 a, a drive transistor Tr2 is connected to the anode ofeach light emission element, and the source of a control transistor Tr1is connected to the gate of the drive transistor Tr2. The drivetransistor Tr2 and the control transistor Tr1 are, for example, FFTs(Field Effect Transistors).

In a case where the contact 3 c of the switch 3 is placed on the contactpoint 3 a so that a voltage is thereby applied between the power supplylines 4 and 5, when the control transistors Tr1 are activated by acontrol signal sent from a control circuit 7, the drive transistors Tr2become conductive, which causes the respective light emission elementsD00 through D23 in the element array 1 a to emit light. When the contact3 c of the switch 3 is placed on the contact point 3 b, switching ismade for the respective light emission elements in the element array 1 bto be activated. That is, as in the first embodiment, this embodimentalso switches the element arrays by controlling the anodes of the lightemission elements.

Since the control transistors Tr1 are connected in series to the drivetransistors Tr2 connected to the respective light emission elements itis possible to select individual light emission elements in therespective element arrays 1 a and 1 b for emitting light. Therefore, itis possible to meet various needs for image formation.

FIG. 5 shows a third embodiment of the invention. The same componentsare designated by the same reference numerals and repetitiveexplanations will be omitted.

In a line head 10 b, switching transistors Tr3 and Tr4 for switchingelement arrays are connected in series to the drive transistors Tr2common to the element arrays 1 a and 1 b. Signal lines 8 and 9 throughwhich selection signals Sel1 and Sel2 for the element arrays are fed. Aninverter INV outputs a selection signal Sel2, which is an invertedselection signal Sel1, to be fed to the signal line 9. Herein, theselection signal Sel2, which is an inverted selection signal Sel1, maybe externally fed to omit the inverter INV.

The positive power supply line 4 is commonly connected to the anodes ofthe respective light emission elements in the element arrays 1 a and 1b. Also, the negative power supply line 5 is connected to the cathodesof the respective light emission elements in the element array 1 a, andthe negative power supply line 6 is connected to the cathodes of therespective light emission elements in the element array 1 b. Thenegative power supply lines 5 and 6 are maintained in a connected stateat a common potential.

When the control transistors Tr1 are activated by a control signal fromthe control circuit 7, the drive transistors Tr2 become conductive. Whenthe selection signal Sel1 is fed to the gates of the transistors Tr3 forswitching the element arrays from the signal line 8 in this state, therespective light emission elements in the element array 1 a areactivated. In this instance, because no selection signal Sel2 is fedfrom the signal line 9, the light emission elements in the element array1 b are kept deactivated.

When the selection signal Sel1 from the signal line 8 is turned off andthe selection signal Sel2 from the signal line 9 is turned on, theswitching transistors Tr3 are shut and transistors Tr4 becomeconductive. The light emission elements in the element array 1 a arethereby deactivated, and instead the light emission elements in theelement array 1 b are activated. In other words, the transistors Tr3 andTr4 are activated by the selection signals Sel1 and Sel2 from the signallines 8 and 9, respectively, to control the switching of the respectiveelement arrays 1 a and 1 b at the cathodes.

Since the switching of the element arrays is performed by thetransistors Tr3 and Tr4, it is possible to achieve a rapid and reliableswitching operation in comparison with a mechanical switch. In addition,since the element arrays are switched by the signals from the signallines without switching the polarities of the power supply lines, it ispossible to prevent the occurrence of an instantaneous voltage changeassociated with the switching, which can in turn prevent damages to thelight emission elements.

In a case where the light emission elements comprise organic EL elementsand the switching transistors Tr3 and Tr4 comprise TFTs (Thin FilmTransistors), it is possible to assemble the line head 10 b using thesame fabrication technique for both the switching transistors and thelight emission elements. The manufacturing costs, therefore, can besaved Also in this embodiment, it is possible to activate or deactivatethe individual light emission elements in the respective element array,by controlling the operation timings of the control transistors Tr1 withthe use of the control signal from the control circuit 7.

FIG. 6 shows a fourth embodiment of the invention. The same componentsare designated by the same reference numerals and repetitiveexplanations will be omitted.

In a line head 10 c, a driver transistor Tr5 is provided for each lightemission element in the element array 1 a, a driver transistor Tr6 isprovided for each light omission element in the element array 1 b, andtransistors Tr7 and Tr8 for switching the element arrays are connectedin series to the control transistors Tr1 common to the element arrays 1a and 1 b.

In this embodiment, while the control transistors Tr1 remain conductiveby a signal from the control circuit 7, either of the selection signalsSel1 and Sel2 from the signal lines 8 and 9, respectively, is fed to thetransistor Tr7 or Tr8 for switching the element arrays. In thisinstance, the driver transistors Tr5 or Tr6 connected to the transistorTr7 or Tr8 are activated, which causes the respective light emissionelements in the element array 1 a or the light emission element 2 to beactivated.

FIG. 7 shows a fifth embodiment of the invention. The same componentsare designated by the same reference numerals and repetitiveexplanations will be omitted.

In a line head 10 d, transistors Tr7 and Tr8 for switching the elementarrays are connected in series to the control transistors Tr1 common tothe element arrays 1 a and 1 b. Also, transistors Tr7 and Tr8 areconnected in series to the drive transistors Tr5 and Tr6 for the lightomission elements in the respective element arrays.

To the signal line 11 are fed selection signals Sel1 through Sel4 toselect the respective light emission elements in the element array 1 a.To the signal line 12 are fed inverted signals of the selection signalsSel1 through Sel4 to select the respective light emission elements inthe element array 1 b. When the selection signals Sel1 through Sel4 areturned on, the inverted signals are turned off, and vice versa.

Any of the selection signals Sel1 through Sel4 is fed to the drains ofthe switching transistors Tr7 corresponding to the respective lightemission elements in the element array 1 a. Also, any of the invertedsignals of the selection signals Sel1 through Sel4 is fed to the drainsof the switching transistors Tr8 corresponding to the respective lightemission elements in the element array 1 b.

While the control transistors Tr1 are activated by a control signal fromthe control circuit 7, the switching transistors Tr7 and the drivetransistors Tr5 become conductive when the selection signals Sel1through Sel4 from the signal line 11 are turned on. The respective lightemission elements in the element array 1 a are thereby activated. Inthis instance, because the inverted signals of the selection signalsSel1 through Sel4 in the signal line 12 are turned off, the respectivelight emission elements in the element array are kept deactivated.

When the inverted signals of the selection signals Sel1 through Sel4 inthe signal line 12 are turned on, the respective light emission elementsin the element array 1 b are activated, and instead the respective lightemission elements in the element array 1 a are deactivated. Also in thisembodiment, by controlling the operation timings of the controltransistors Tr1 with the use of the control signal from the controlcircuit 7, it is possible to select individual light emission elementsin the element arrays 1 a and 1 b for emitting light.

FIG. 8 shows a sixth embodiment of the invention. The same componentsare designated by the same reference numerals and repetitiveexplanations will be omitted.

In a line head 10 e, transistors Tr9 and Tr10 are for switching theelement arrays With the switching transistors Tr9, the gates areconnected to the signal line 11 and the drains are connected to thesources of the control transistors Tr1. With the switching transistorsTr10, the gates are connected to the signal line 12 and the drains areconnected to the sources of the control transistors Tr1. The transistorsTr9 and Tr10 comprise FETs of the same channel (N channel in thisembodiment).

The transistors Tr9 and Tr10 are connected in series to the controltransistors Tr1. While the control transistors Tr1 remain conductive bya control signal from the control circuit 7, the respective lightemission elements in the element array 1 a or the element array 1 b areactivated by enabling either the signal line 11 or 12.

In this embodiment, it is possible to select individual light emissionelements in the element arrays 1 a and 1 b for emitting light, bycontrolling the operation timings of the control transistors Tr1 by thecontrol circuit 7. This embodiment is different from the fifthembodiment in that the output signals of the control transistors Tr1 arefed to the drains of the transistors Tr9 and Tr10, and that selectionsignals are fed to the gates of the switching transistors Tr9 and Tr10.In short, these embodiments are different in the way by which thecontrol transistors Tr1 are connected to the transistors Ir9 and Tr10.

FIG. 9 shows a seventh embodiment of the invention. The same componentsare designated by the same reference numerals and repetitiveexplanations will be omitted.

In a line head 10 f, the transistors Tr9 and Tr11 for switching theelement arrays comprise FETs of different channels. In this embodiment,the transistors Tr9 comprise N-channel FETs and the transistors Tr11comprise P-channel FETs.

To the gates of a pair of the transistors Tr9 and Tr11 are fed signalsSel1 through Sel4 from the same data line 11. Hence, when one transistorTr9 is turned on, the other transistor Tr11 is turned off, and viceversa. In other words, when the signals Sel1 through Sel4 are turned on,all the light emission elements in the element array 1 a are activatedand all the light emission elements in the element array 1 b are keptdeactivated. When the signals Sel1 through Sel4 are turned off, all thelight emission elements in the element array 1 a are kept deactivatedand all the light emission elements in the element array 1 b areactivated. Therefore, the data line to feed the inverted signals of thesignals Sel1 through Sel4 can be omitted.

FIG. 11 shows an eighth embodiment of the invention. The same componentsare designated by the same reference numerals and repetitiveexplanations will be omitted.

In this embodiment, one of the four element arrays 1 a through 1 d shownin FIG. 10 is subsequently activated. Specifically, all the lightemission elements in the element array 1 a are activated from the timepoints 0 to tx. All the light omission elements in the element arrays 1b through 1 d are kept deactivated. At the time point tx, the elementarray 1 a is deactivated, and switching is made for the element array 1b to be activated. Thereafter, at the time point ty, the element array 1b is deactivated and switching is made for the element array 1 c to beactivated.

At the time point tz, the element array 1 c is deactivated and switchingis made for the light emission element 1 d to be activated. At the timepoint tw, the element array 1 d is deactivated. In a case wherepredetermined luminance has been restored in the element array 1 a dueto a drop in temperature of the respective light emission elements, itis possible to activate the element array 1 a again at the time pointtw. Also, it is possible to activate the respective element arrays 1 bthrough 1 d again after a predetermined time period since they arerespectively deactivated.

Accordingly, it is possible to prevent the decrease in luminance causedwhen a single element array is kept activated over a long time period,thereby prolonging the lifetime of the line head.

In this embodiment, an allowable continuous driving time period isstored in advance in the memory 25 shown in FIG. 2.

The host controller 23 judges whether the activated time period of oneelement array exceeds the allowable time period stored in the memory 25.Upon judging that the activated time period of the one element array hasexceeded the allowable time period, the host controller 23 sends asignal to the switching circuit 21 to activate another element arrayformed in the line head 10.

The counter 24 counts the time period that one element array isactivated for the above judgment. Since the luminance decrease of thelight emission element can be judged on the basis of the number ofprinted dots or print media, the counter 24 and the memory 25 can beconfigured so as to set various parameters, such as the number of printdots or print media, to be used for the above judgment.

Specifically, in a case where the number of print dots is given as aparameter, the allowable number of the print dots is registered in thememory 25. The counter 24 is formed as a dot counter, so that when thecount number in the dot counter exceeds the allowable number registeredin the memory 25, the host controller 23 outputs a switching signal ofthe element arrays to the switching circuit 21.

When the number of print media is given as a switching parameter for theelement arrays, a switching signal is outputted, for example, each timea sheet of print medium has been printed. Alternatively, when a verticalsync signal (Vsync) is given as a switching parameter for the elementarrays, the number of pulses is counted. It should be noted that in thecase of color print, the counter 24 and the memory 25 are provided foreach color.

FIG. 12 shows a ninth embodiment of the invention. The same componentsare designated by the same reference numerals and repetitiveexplanations will be omitted.

The element array 1 a is provided for the normal operation and theelement array 1 b is provided for the backup operation. In the event ofa failure in any of the light emission elements D00 through D23 in theelement array 1 a, a voltage is applied to the respective light emissionelements D50 through D73 in the element array 1 b by the switch 3 forlight emitting operations to be performed.

Shift registers 11, 12, 13 controls the light emission elements D00through D23 block by block. An output signal C0 of the shift register 11controls a block A including the light emission elements D00 throughD03. An output signal C1 of the shift register 12 controls a block Bincluding the light emission elements D10 through D13. An output signalC2 of the shift register 13 controls a block C including the lightemission elements D20 through D23.

A start pulse SP is inputted to a data terminal D of the shift register11 from a signal line 17. A clock signal CK is inputted to each of theshift registers 11 through 13 from a signal line 18. Data signals Dat0through Dat3 are fed from a signal line 70 to the respective lightemission elements. The output signal C0, outputted from an outputterminal Q of the shift register 11, is applied via a signal line C0 ato the gates of the respective control transistors Tr1 connected to thelight emission elements D00 through D03. The output signal C1, outputtedfrom an output terminal Q of the shift register 12, is applied via asignal line C1 a to the gates of the respective control transistors Tr1connected to the light emission elements D10 through D13. The outputsignal C2 outputted from an output terminal Q of the shift register 13,is applied via a signal line C2 a to the gates of the respective controltransistors Tr1 connected to the light emission elements D20 throughD23.

In short, when the output signals C0 through C2 are at the high level(hereinafter, referred to as the H level), signals are applied to thegates of the respective control transistors Tr1 that control the lightemission elements in the corresponding blocks. All the light emissionelements are connected in parallel between the power supply line 4 towhich a positive voltage VDD is applied and the negative power supplyline 5. Sine the light emission elements in each element array aredivided into blocks to be activated or deactivated, an area at anexposure operation is executed can be controlled as required.

The data signals Dat0 through Dat3 are fed to the drains of therespective control transistors Tr1. Hence, when the data signals Dat0through Dat3 are fed to the control transistors Tr1 for the lightemission elements selected by the block selection signals, the drivetransistors Tr2 connected to these control transistors Tr1 becomeconductive, which causes the corresponding light emission elements to beactivated. For example, in the case of the block A, the data signalsDat0 through Dat3 are fed to the control transistors Tr1 thatrespectively control the light emission elements D00 through D03. Inother words, the data signals Dat0 through Dat3 serve as selectionsignals to select individual light emission elements within one block.That is, it is possible to select at least one individual light emissionelement to be activated.

Concrete operations of the respective light emission elements will nowbe described with reference to the timing chart of FIG. 13. Assume thata DC voltage is applied between the positive and negative power supplylines 4 and 5 connected to the respective light emission elements ofFIG. 12. In this timing chart, an operation time of the light emissionelements is divided into 8 segments of time period. The length of eachsegment is set, for example, to 10 μs. When the start pulse SP isinputted to the data terminal D of the shift register 11, the startpulse SP is captured in synchronous with the rising of the clock signalCK, and transferred sequentially from the shift registers 11 to 13.

Herein, intervals of the signal SP are set in such a manner that anoutput of any one of the shift registers 11 to 13 alone is at the Hlevel. For example, in the segment 1, the start pulse SP is inputted tothe shift register 11 at the time point 0, at which the output signal C0of the shift register 11 shifts to the H level because the clock signalCK is at the H level at the timing “u”. The start pulse SP is thentransferred to the shift register 12 at the timing “v” at which the nextclock signal CK shifts to the H level, and the output signal C1 therebyshifts to the H level. Further, the start pulse SP is transferred to theshift register 13 at the timing “w” at which the next clock signal CKshifts to the H level, and the output signal C2 thereby shifts to the Hlevel.

Outputs from the respective shift registers are connected to the controltransistors Tr1 in each block. As is shown in FIG. 13, the signals C0through C2 to select the respective blocks are applied to the controltransistors Tr1 with a time difference. Hence, when the switches of thecontrol transistors Tr1 in a given block are turned on, the switches ofgroups of the control transistors Tr1 in the other blocks are kept off.

In the selected block, the control transistors Tr1 are turned on so asto establish a condition that the data signals Dat0 through Dat3 can befed to the gates of the drive transistors Tr2. The potentials of thegates of the drive transistors Tr2 are thus determined according to thestates (H level or L level) of the data signals Dat0 through Dat3, andin turn the on/off states of the drive transistors Tr2 in the selectiveblock are determined.

When a given block that is selected will no longer be selected, the datasignals Dat0 through Dat3 will not be fed to the gates of the drivetransistors Tr2 in the block that has boon selected. However, becausethe potentials of the gates of the drive transistors Tr2 are held at thepotentials when the block was selected due to a parasitic capacitance,the on/off states of the drive transistors Tr2 are maintained. Thisstate of the drive transistors Tr2 is maintained until the block isselected again.

When the block in the deselected state is selected at the next timing,the on/off states of the drive transistors Tr2 in the block aredetermined according to the states of the data signals Dat0 throughDat3. The operations described as above are repeated sequentially forall the blocks. Hence, the activation time period can be controlled onthe basis of this repetitive cycle.

Referring to FIG. 13, the block A is selected at the time point 0 in thesegment 1. The block B is selected next at the time point ta, and theblock C at the time point tc. For the segments 2 through 8, any of thecorresponding blocks A through C is selected in the same manner as abovewhen the output signals C0 through C2 of the shift registers 11 through13, respectively, are at the H level. Each of the vertically extendinghatched areas indicates that the corresponding block is selected. Eachof the horizontally extending hatched areas indicates that thecorresponding light emission element is activated.

The data signals Dat0 through Dat3 select individual light emissionelements within the respective blocks A through C as described above.For example, within the block A, the correspondence between the datasignals and the light emission elements is as follows: Dat0(D00),Dat1(D01), Dat2(D02), and Dat3(D03). Fine lines (F) indicate thecorrespondence between the data signals and the light emission elementsin the respective blocks as described above.

For the block A, in the segment 1, when the block selection signal C0 isat the H level at the time point 0, the data signals Dat0 and Dat3 shiftto the H level, and the light emission elements D00 and D03 areactivated. When the block selection signal C0 shifts to the low level(hereinafter, referred to as the L level) at the time point to, thevoltage is maintained due to a parasitic capacitance between the gateand the source in each corresponding control transistor Tr1. The lightemission elements D00 and D03 thus remain activated.

The block selection signal C0 and the data signal Dat3 shift to the Llevel at the time point tc, at which the block selection signal C1shifts to the H level and the block B is thereby selected. However,because the drive transistor Tr2 remain conductive due to the parasiticcapacitance between the gate and the source in the control transistorTr1, the light emission element D03 remains activated.

The block selection signal C0 shifts to the H level at the point ts, andthe on/off state of the drive transistor Tr2 connected to the lightemission element D03 is determined according to the state of the datasignal Dat3. At the time point ts, the control transistor Tr1 is turnedoff because the data signal Dat3 is at the L level, and the drivetransistor Tr2 is also turned off. The light emission element D03 isthereby deactivated.

The block selection signal C0 and the data signal Dat2 shift to the Hlevel at the time point te, and the light emission element D02 isthereby activated. The data signal Dat2 shifts to the L level at thetime point tr; however, the activated state is maintained until the timepoint ts, that is, the timing at which the block selection signal C0shifts to the H level again. The light omission element D03 isdeactivated at the time point ts as described above.

The data signal Dat0 shifts to the L level at the time point tu, and thelight emission element D00 is thereby deactivated. As is shown in anenlarged view of the portion (E), the switching of the on/off states ofthe light emission element D00 takes place in the selection period ofthe block selection signal C0. The switching of the on/off states of theother light emission elements also takes place in the selection periodsof the block selection signals (although not explicitly shown in thedrawing).

Now, by referring to the operation for the block B, because the datasignal Dat0 is at the H level when the block selection signal C1 is atthe H level at the time point ta in the segment 1, the light emissionelement D10 is activated. Although the light emission element D10 is inan indefinite state from the time points 0 to ta, it is placed in theactivated state at the time point ta as described above.

In the segment 3, the block selection signal C1 shifts to the H leveland the data signal Dat1 shifts to the H level at the time point tf, andthe light emission element D11 is thereby activated. Also, because thedata signal Dat3 is at the H level at the time point tf, the lightemission element D13 is also activated. Because the block selectionsignal C1 shifts to the H level at the time point th, the light emissionelement D13 is activated or deactivated according to the state of thedata signal Dat3. Because the data signal Dat3 is at the L level in thisinstance, the light emission element D13 is deactivated.

At the time point tr, because the block selection signal C1 shifts tothe H level and the data signal Dat1 shifts to the L level, the lightemission element D11 is deactivated. Because the data signal Dat3 is atthe H level at the time point tr, the light emission element D13 isactivated. The block selection signal C1 shifts to the L level and thedata signal Dat0 also shifts to the L level at the time point tu.However, as has been described, the drive transistor Tr2 remainsconductive, and the light emission element D10 thus remains activateduntil the time point tv as is indicated by G in the drawing.Descriptions for the operations of the respective light emissionelements in the block C are omitted.

FIG. 14 is an explanatory view, showing the timing chart of FIG. 3 inthe form of a table. In this table, a circle indicates that thecorresponding block is selected. In the “data” section, “1” indicatesthe H level, and “0” indicates the L level of the data signal Dat 0through Dat 3.

In the “light emission element” section, each of the verticallyextending hatched areas indicates that the corresponding light emissionelement is activated (i.e., corresponding to the horizontal hatchedareas in FIG. 13). An asterisk indicates an indefinite state of thelight emission element. An arrow indicates that whether the lightemission element is activated or deactivated is determined by the on/offstate of the data signal. A blank cell indicates a state where thepreceding state is maintained. In this manner, in this embodiment, eachof the plural element arrays is divided into plural blocks, andindividual light emission elements in the respective blocks are selectedand controlled as long as necessary. In other words, because twoselecting stages are provided for individual light emission elements inthe element arrays, it is possible to address image formation processingof various modes. The applications of the line head, therefore, can bebroadened.

In this embodiment, since the shift registers 11 through 13 can beformed on a single substrate together with the element arrays 1 a and 1b, the control transistors Tr1, and the drive transistors Tr2. A compactline head can be thereby achieved.

FIG. 15 shows a tenth embodiment of the invention. The same componentsare designated by the same reference numerals and repetitiveexplanations will be omitted.

In this embodiment, the element array 1 a is used for forming a binarydata image such as character information, and the element array 1 b forforming a gradation data image such as photographic information.Specifically, all the light emission elements in the element array 1 aare activated during the time period 0 to ta to form the binary dataimage. All the light emission elements in the element array 1 b are keptdeactivated.

When luminance decreases below the predetermined value due thecontinuous activation of the element array 1 a, the element array 1 a isdeactivated at the time point ta Incidentally, switching is made for theelement array 1 b to be activated, so that the gradation data image isformed. At the time point tb, the element array 1 b is deactivated whilethe element array 1 a is activated to form the binary data image for ashort time until the time point tc. Because the activation time periodof the element array 1 a is short and only the binary data image isformed, deterioration of the obtained image quality poses no practicalproblem.

Generally, the accuracy in image formation, when an image is formed withdecreased luminance of the light emission elements, differs between acase where the binary data image is formed and a case where thegradation data image is formed. In the former case, even when luminanceof the light emission elements decreases slightly, a degree ofdeterioration of the obtained image quality is smaller then in thelatter case.

Thereafter, the gradation data image is formed by the element array 1 buntil the time point td, and the binary data image is formed by theelement array 1 a during the time period td to te. Herein, because acertain time period has passed since the respective light emissionelements in the element array 1 a were deactivated at the time point ta,the temperature has dropped and luminance has been restored. Hence, thebinary data image is formed during the time period td to te, which islonger than the time period tb to tc. From the time points te to tf, thegradation data image is formed by the element array 1 b. Because thetemperature of the light emission element in the element array 1 a hassufficiently dropped at the time point tf and the luminance thereof hasbeen sufficiently restored, the binary data image is formedcontinuously.

According to the configuration of this embodiment, it is possible toprevent deterioration of the print quality due to the decrease inluminance of the light emission elements in comparison with a case whereboth the binary data image and the gradation data image are formedcontinuously by a single element array. It is also possible to prolongthe lifetime of the light emission elements and the line head.

The switching timings of the element arrays as shown in FIG. 15 are setby taking various factors into account. In other words, optimal timingsare set on the basis of a temperature characteristic of the lightemission element materials, a heat-releasing characteristic of the panelof the line head to which the light emission elements are attached, etc.In this embodiment, two element arrays are switched according to imageinformation including binary data and gradation data. However, it may beconfigured in such a manner that three or more element arrays are usedby being switched at adequate timings.

FIG. 16 shows an eleventh embodiment of the invention. The samecomponents are designated by the same reference numerals and repetitiveexplanations will be omitted.

In this embodiment, the element array 1 a is first activated to formboth the binary data image and the gradation data image, and the elementarray 1 b is not used. However, after a predetermined time has passed,the element array 1 a is used to form only the binary data image whilethe element array 1 b is used to form only the gradation data image. Asin the tenth embodiment, it is configured that the element arrays 1 aand 1 b are activated alternately.

Specifically, all the light emission elements in the element array 1 aare activated during the time period 0 to tp to form the binary dataimage and the gradation data image. All the light emission elements inthe element array 1 b are kept deactivated. When luminance decreasesbelow a predetermined value due to the continuous activation of theelement array 1 a, the element array 1 a is deactivated at the timepoint tp, and switching is made for the element array 1 b to form thegradation data image. At the time point tq, the element array 1 b isdeactivated and the element array 1 a is activated to form the binarydata image for a short time until the time point tr. Because theactivation time period of the element array 1 a is short and only thebinary data image is formed, deterioration of the obtained image qualityposes no practical problem.

Thereafter, the gradation data image is formed by the element array 1 buntil the time point ts, the binary data image is formed by the elementarray 1 a during the time period ts to tu, and the gradation data imageis formed by the element array 1 b during the time period tu to tv, in arepetitive manner. At the time point tv, because the temperature of thelight emission elements in the element array 1 a has sufficientlydropped and the luminance thereof has been sufficiently restored, thebinary data image and the gradation data image are formed again.

FIG. 17 shows a peripheral configuration of the line head of the tenthand eleventh embodiments. In this embodiment, a counter 24 a and amemory 25 a are associated with the element array 1 a. A counter 24 band a memory 25 b are associated with the element array 1 b.

The counter 24 a counts the activated time period of the element array 1a, and stores the counted value into the memory 26 a. In the memory 26a, switching timing data of the element array 1 a is stored in advance.Similarly, the counter 24 b counts the activated time period of theelement array 1 b, and stores the counted value into the memory 25 b. Inthe memory 25 b, switching timing data of the element array 1 b isstored in advance. The host controller 23 judges whether the activatedtime periods of the element arrays 1 a and 1 b reach the switchingtimings stored in the memories 25 a and 25 b, respectively. Upon judgingthat the switching timing has reached for the element array that isactivated, the host controller 23 sends a signal to the switchingcircuit 21 to switch to the activated element array to another one.

In this case, the host controller 23 controls the counters 24 a and 24 band the memories 25 a and 25 b. However, a controller such as a CPU forcontrolling the counters 24 a, 24 b and the memories 25 a, 25 b may beprovided in the control section 20, such that the controller is formedon a substrate on which the switching circuit 21 is provided. In thiscase, the configuration of the control system can be compact and thehost controller 23 is not involved. Hence, the wiring can be shorter,which can in turn increase the processing speed.

The line head according to any one of the above embodiments can beapplied to not only a monochromatic printer, but also a four-cycle typefull color printer, and a tandem type full color printer. There will bedescribed cases where the line head is incorporated in the full colorprinter.

FIG. 18 shows a tandem-type image forming apparatus in which tourexposure heads 10 k, 10C, 10M and 10Y having the same configurationcomprising a number of organic EL elements are disposed so as to faceexposure positions on four photosensitive drums (image carriers) 41 k,41C, 41M, and 41Y having the same configuration.

In the image forming apparatus, an intermediate transfer belt 50stretched over a driving roller 51, a follower roller 52, and a tensionroller 53 and circulated in a direction indicated by an arrow in thedrawing (counterclockwise direction) while being tensed by the tensionroller 53. The photosensitive drums 41K, 41C, 41M, and 41Y, each havinga photosensitive layer on the outer peripheral surface thereof, areplaced at regular intervals with respect to the intermediate transferbelt 50. Capitals K, C, M and Y appended to the reference numerals standfor black, cyan, magenta, and yellow, respectively, and thereby indicatethat they are photosensitive drums for black, cyan, magenta, and yellow,respectively. The same can be said for other members. The photosensitivedrums 41K, 41C, 41M, and 41Y are rotated in a direction indicated byarrows in the drawing (clockwise direction) in synchronous with thecirculation of the intermediate transfer belt 50.

On the periphery of the respective photosensitive drums 41 (K, C, M, andY) are provided corona chargers 42 (K, C, M, and Y) for uniformlycharging the outer peripheral surfaces of the respective photosensitivedrums 41 (K, C, M, and Y), and the exposure heads 10 (K, C, M, and Y)for scanning the outer peripheral surfaces that have been chargeduniformly by the corona chargers 42 (K, C, M, and Y), in synchronouswith the rotations of the photosensitive drums 41 (K, C, M, and Y) toform electrostatic latent images thereon.

The image forming apparatus further comprises: developers 44 (K, C, M,and Y) to develop the electrostatic latent images as visible images withtoner; primary transfer rollers 45 (K, C, M, and Y) for sequentiallytransferring the toner images developed by the developers 44 (K, C, M,and Y) onto the intermediate transfer belt 50 (primary transfer); andcleaners 46 (K, C, M, and Y) for removing toner remaining on thesurfaces of the photosensitive drums 41 (K, C, M, and Y) after theprimary transfer.

In each of the respective exposure heads 10 (K, C, M, and Y), theorganic EL elements are arrayed in a direction parallel to a generatrixof the photosensitive drums 41 (K, C, M, and Y). The peak energywavelengths of light emitted from the respective exposure heads 10 (K,C, M, and Y) are almost matched with the peak sensitivity wavelengths ofthe photosensitive drums 41 (K, C, M, and Y).

In the developer 44 (K, C, M, and Y), non-magnetic one-component toneris transported to a developing roller by a feeding roller. The thicknessof the toner adhering to the surface of the developing roller isrestricted by a control blade. The developing roller is brought intocontact with or pressed against the photosensitive drum 41 (K, C, M, andY) to adhere the toner thereon according to the potential levels of thephotosensitive drums 41 (K, C, M, and Y). A toner image is thusdeveloped.

The respective toner images of black, cyan, magenta, and yellow aresequentially transferred onto the intermediate transfer belt 50 byprimary transfer biases applied to the primary transfer rollers 45 (K,C, M, and Y), and are sequentially superimposed on the intermediatetransfer belt 50 to form a full color toner image, which is secondarytransferred into a recording medium P, such as paper, by a secondarytransfer roller 66 at a secondary transfer position. The toner image isthen fixed onto the recording medium P by passing through a pair offixing rollers 61. The recording medium P is finally ejected by a pairof ejection rollers 62 onto a tray 68 formed at the top portion at theapparatus.

The image forming apparatus further comprises: a feeding cassette 63 inwhich a number of recording media P are held in a stacked manner; apickup roller 64 to feed the recording media P from the feeding cassette63 one by one; a pair of gate rollers 65 to regulate the feeding timingof a recording medium P to the secondary transfer position; and acleaning blade 67 for removing toner remaining on the surface of theintermediate transfer belt 50 after the secondary transfer.

In a case where the above-described line head 10 is incorporated in theimage forming apparatus as the exposer, it is possible to downsize theapparatus in comparison with a case where a laser scanning opticalsystem is used.

FIG. 19 shows a four-cycle type image forming apparatus 160 roughlycomprising a rotary-type developer 161; a photosensitive drum (imagecarrier) 165; an exposure head 10 comprising organic EL elements; anintermediate transfer belt 169; a sheet transportation path 174; and asheet feeding tray 178.

In the developer 161, a rotary 161 a rotates about an axis 161 b in adirection indicated by an arrow A. The interior of the rotary 161 a isdivided into four segments, in which are respectively formed imageforming units for four colors, including yellow (Y), cyan (C), magenta(M), and black (K). Developing rollers 162 a through 162 d arerespectively placed in the image forming units for four colors, torotate in a direction indicated by an arrow B. Toner supply rollers 163a through 163 d is rotated in a direction indicated by an arrow C.Control blades 164 a through 164 d to restrict the thickness of toner onthe developing rollers 162 a through 162 d to a predetermined thickness.

The image forming apparatus 160 further comprises a primary transfermember 166; and a charger 168. The photosensitive drum 165 is rotated byan unillustrated driving motor such as a step motor, in a directionindicated by an arrow D, which is opposite to the rotary direction ofthe developing roller 162 a.

The intermediate transfer belt 169 is stretched over a driving roller170 a and a follower roller 170 b. The driving roller 170 a is linked toa driving motor of the photosensitive drum 165, so that the intermediatetransfer belt 169 is circulated in a direction indicated by an arrow E,which is opposite to the direction of the photosensitive drum 165.

The sheet transportation path 174 is provided with a plurality oftransportation rollers and a pair of ejection rollers 176. The tonerimage carried by the intermediate transfer belt 169 is transferred ontoone surface of a sheet of paper by a secondary transfer roller 171 atthe secondary transfer position.

The sheet of paper, onto which the toner image has been transferred asdescribed above, is then subjected to fixing processing at a fuser. Thefuser is provided with a heating roller 172 and a press roller 173.After the fixing processing, the sheet of paper is pulled into the pairof ejection rollers 176 to travel in a direction indicated by an arrowF. When the pair of ejection rollers 176 rotates in an inverse directionfrom this state, the sheet of paper inverts the direction and travels ina direction indicated by an arrow G through a double-sided printtransportation path 175. Numeral 177 denotes an electrical equipmentbox, and numeral 179 denotes a pickup roller provided at the outlet ofthe feeding tray 178.

For the sheet transportation path, for example, a low-speed brushlessmotor is used at the driving motor to drive the transportation rollers.Also, a step motor is used for the intermediate transfer belt 169 due toa need to correct color shift or the like. These motors are controlledby signals from an unillustrated controller.

In the state shown in the drawing, an electrostatic latent image ofyellow (Y) is formed on the photosensitive drum 165, and an image ofyellow is formed on the photosensitive drum 165 when a high voltage isapplied to the developing roller 162 a. When images of yellow on theback surface and the front surface are entirely carried over onto theintermediate transfer belt 169, the developing rotary 161 a rotates by90 degrees in a direction indicated by the arrow A.

The intermediate transfer belt 169 rotates once and returns to theposition of the photosensitive drum 165. Images of cyan (C) on twosurfaces are then formed on the photosensitive drum 165, which arecarried over to be superimposed on the images of yellow being carried onthe intermediate transfer belt 169. Thereafter, the processing isrepeated in the same manner, so that the developing rotary 161 rotatesby 90 degrees and the intermediate transfer belt 169 rotates once afterthe images are carried over.

For images of four colors to be carried over, the intermediate transferbelt 169 rotate four times, after which the rotational position iscontributed for the image to be transferred onto a sheet of paper at theposition of the secondary transfer roller 171. A sheet of paper fed fromthe feeding tray 178 is transported through the transportation path 174,and the color image is transferred onto one surface of the sheet ofpaper at the position of the secondary transfer roller 171. The sheet ofpaper bearing the transferred image on one surface is inverted by thepair of ejection rollers 176 as described above, and stands by in thetransportation path. Subsequently, the sheet of paper is transported tothe position of the secondary transfer roller 171 at the adequatetiming, and the color image is transferred onto the other surface. Ahousing 180 is provided with an exhaust fan 181.

Although the present invention has been shown and described withreference to specific preferred embodiments, various changes andmodifications will be apparent to those skilled in the art from theteachings herein. Such changes and modifications as are obvious aredeemed to come within the spirit, scope and contemplation of theinvention as defined in the appended claims.

1. A line head, comprising: a plurality of element arrays arranged in asecondary scanning direction, each array including a plurality of lightemission elements arrayed in a primary scanning direction; and a firstswitcher, which activates the element array in at least one of theelement arrays while deactivating the others, wherein at least one ofthe element arrays is used for a backup purpose.
 2. The line head as setforth in claim 1, wherein the first switcher comprises switchingtransistors.
 3. The line head as set forth in claim 2, wherein: each ofthe element arrays is associated with one of the switching transistors;and the first switcher simultaneously generates a first signal foractivating the switching transistor and a second signal for deactivatingthe switching transistor, so that the first signal is inputted to theswitching transistor for the at least one element arrays to beactivated, while the second signal is inputted to the switchingtransistor for the others to be deactivated.
 4. The line head as setforth in claim 2, wherein: the element arrays includes at least onefirst element array and at least one second element array; the switchingtransistors includes a first transistor associated with the firstelement array and a second transistor associated with the second elementarray and having a conductive type which is opposite to the firsttransistor; and the first switcher inputs a common signal to the firsttransistor and the second transistor, so that one of the firsttransistor and the second transistor is activated while the other one isdeactivated.
 5. The line head as set forth in claim 1, wherein each ofthe light emission elements is provided as either an organicelectroluminescence element or a light emitting diode.
 6. The line headas set forth in claim 5, wherein the first switcher controls a potentialat an anode of each of the light emission elements.
 7. A line head,comprising: a plurality of element arrays arranged in a secondaryscanning direction, each array including a plurality of light emissionelements arrayed in a primary scanning direction; and a first switcher,which activates the element array in at least one of the element arrayswhile deactivating the others, wherein: each of the element arrays isdivided into a plurality of blocks; and the line head further comprisesa second switcher which activates the light emission elements in atleast one of the blocks in the at least one element array activated bythe first switcher.
 8. The line head as set forth in claim 7, furthercomprising a third switcher which activates at least one of the lightemission elements in the at least one block activated by the secondswitcher.
 9. A line head, comprising: a plurality of element arraysarranged in a secondary scanning direction, each array including aplurality of light emission elements arrayed in a primary scanningdirection; a first switcher, which activates the element array in atleast one of the element arrays while deactivating the others; and asecond switcher which activates at least one of the light emissionelements in the at least one element array activated by the firstswitcher.
 10. An image forming apparatus, comprising: an image carrier,having a photosensitive surface; a line head, comprising: a plurality ofelement arrays arranged in a secondary scanning direction, each arrayincluding a plurality of light emission elements arrayed in a primaryscanning direction, to emit light for forming an electrostatic latentimage on the photosensitive surface; and a first switcher, whichactivates the element array in at least one of the element arrays whiledeactivating the others; and a developer, which develops the latentimage as a visible image with toner, wherein at least one of the elementarrays is used for a backup purpose.
 11. An image forming apparatus,comprising: an image carrier, having a photosensitive surface; a linehead, comprising: a plurality of element arrays arranged in a secondaryscanning direction, each array including a plurality of light emissionelements arrayed in a primary scanning direction; and a first switcher,which activates the element array in at least one of the element arrayswhile deactivating the others; and a developer, which develops thelatent image as a visible image with toner, wherein: each of the elementarrays is divided into a plurality of blocks; and the line head furthercomprises a second switcher which activates the light emission elementsin at least one of the blocks in the at least one element arrayactivated by the first switcher.
 12. An image forming apparatus,comprising: an image carrier, having a photosensitive surface; a linehead, comprising: a plurality of element arrays arranged in a secondaryscanning direction, each array including a plurality of light emissionelements arrayed in a primary scanning direction; a first switcher,which activates the element array in at least one of the element arrayswhile deactivating the others; a second switcher, which activates atleast one of the light emission elements in the at least one elementarray activated by the first switcher; and a developer, which developsthe latent image as a visible image with toner.